USB: serial: kfifo_len locking

[Johan Hovold <jhovold@gmail.com>]

Hi Stefani,

I noticed that the locking that used to protect kfifo_len in
usb_serial_generic_chars_in_buffer was removed when the kifo api changed
to not use internal locking (c1e13f25674ed564948ecb7dfe5f83e578892896 -- 
kfifo: move out spinlock). 

Was this intentional?

I found a related discussion here

	http://lkml.org/lkml/2009/12/18/433

where you seem to say that no such locking is required as long as
kfifo_reset is never called (and that one could use kfifo_reset_out
instead)?

However, kfifo_reset was still being called when the locking was removed
and not until later was it changed to kfifo_reset_out
(119eecc831a42bd090543568932e440c6831f1bb -- Fix usb_serial_probe()
problem introduced by the recent kfifo changes).  

Does this last change imply that no locking in
usb_serial_generic_chars_in_buffer is required? If this is the case,
perhaps such locking guidelines could be added to kfifo.h?

Thanks,
Johan